The assembly of flip chip devices requires electrical connection or bonding of solder bumps on a die to leads on a substrate or printed circuit board. One method of achieving the electrical connections is via a thermal compression technique whereby the substrate and die are compressed together under a heated condition. Solder bumps on the die are aligned with leads on the substrate and then forced together via a compressive force. Ideally, the resulting connection is a wet connection that is free of traps. In the formation of a wet connection molten solder on the solder bump flows to or into the lead. A “wet connection” as used herein refers to a connection in which a good alloy bond is formed between the solder bump and lead.
Traps may form proximate the bond or between the bumps and the leads and may hold materials, such as non-conductive pastes used in the fabrication of the devices. These traps may reduce the bond area and weaken the connection between the die and the substrate. They may also hold impurities that are not meant to be in the finished flip chip device.
Further complications arise in flip chip fabrication when the electrical connection between the solder bumps and the leads are not wet. Non-wet bonds or connections do not conduct electricity well and may not conduct electricity at all. Additionally, a non-wet bond is more likely to suffer premature failure.